1. Field of the Invention
The present invention is related to a frequency/phase comparator, and more particularly to color subcarrier frequency comparator which may be employed to calibrate oscillators used as frequency standards.
2. Description of the Prior Art
There generally exists two methods of measuring or calibrating oscillators which are utilized as frequency standards. One method utilizes a digital counter to readout the frequency difference directly. The foregoing, however, is generally limited to an accuracy of eight or nine digits. Another accepted method is to compare the phase of a known precise received signal with that generated by the local oscillator to be calibrated, as is done in most of the broadcast type comparison systems available today. All broadcast calibration systems suffer from degradation of accuracy due to variations of the propagation path. Ionospheric path changes limit the accuracy of HF radio (2.5 - 25 MHz) to about .+-. 1 .times. 10.sup..sup.-8 under the best conditions. VLF and LF broadcasts in the frequency range 10 to 100 KHz are less affected by propagation conditions and are used when calibration accuracy requirements exceed .+-. 1 .times. 10.sup.116 8. Most standards laboratories in the United States monitor the phase of the 60 KHz carrier broadcast by radio station WWVB. In order to calibrate an oscillator to .+-. 1 .times. 10.sup..sup.-10, the user must typically make phase recordings of the WWVB signal for 24 hours. Considerable time and operator skill are required to interpret the phase recordings.
It would therefore be desirable to have an apparatus for accurately calibrating an oscillator to an accuracy of at least .+-. 1 .times. 10.sup.10 in about 100 seconds, with relatively inexpensive components.